The present invention relates to a method of forming an optical fiber in which there is an abrupt change in refractive index between two adjacent layers.
Fibers having an abrupt change in refractive index between two different layers, e.g. between the core and the next adjacent layer, have been made by the inside vapor deposition (IVD) process. In accordance with the IVD process, reactants flow through a glass substrate tube while a moving hot zone repeatedly traverses the tube in the direction of reactant flow. Glass particles produced in the hot zone travel downstream and deposit on the tube. These particles are consolidated or fused to form a glass layer during each pass of the hot zone. Thus, there is a minimal diffusion of dopant from the core layers to the adjacent cladding layers.
The outside vapor deposition (OVD) process, whereby glass particles, often referred to as soot, are directed laterally or axially onto a starting member, provides a higher deposition rate and larger preform size than the IVD process. A fiber having a core and cladding is usually formed by depositing a cylindrically-shaped preform of core glass particles and cladding glass particles and consolidating that preform to form a draw blank which can be drawn into a fiber. During the consolidation process, dopant from the core portion of the porous preform can migrate through the pores to the cladding portion, thereby creating a dopant-depleted region at the edge of the core and a corresponding dopant-rich region in the adjacent cladding. The resultant core-cladding interface is not sufficiently abrupt for certain applications.